Starting in the early 1960's a unique printing technology has developed known generally as ink jet printing. The first devices of this type produced a continuous stream of tiny ink droplets, some of which were electrostatically deflected to desired locations on an adjacent record medium and others of which were similarly deflected to an ink collection device of some sort, usually for reuse. U.S. Pat. Nos. 3,298,030 issued to Lewis and Brown and 3,596,275 issued to Sweet disclose early versions of such continuous streaming ink jet printers. In the late 1960's an improved type of ink jet technology was developed in which tiny droplets were produced only when needed and then directed to their associated locations on an adjacent record medium, thus eliminating the need for any ink collection system for unused droplets. Moreover, when arrays of such jets are used, electrostatic deflection of the droplets is not required, either to print alphanumerics or other images, or to remove unneeded droplets. Single jets, however, still require electrostatic deflection to print alphanumerics. U.S. Pat. Nos. 3,683,212 issued to Zoltan and 3,832,579 issued to Arndt disclose such drop-on-demand ink jet printers.
While the drop-on-demand technology represented a substantial simplification of the earlier continuous streaming ink jet printers, the reduction in complexity was achieved at some cost in the form of reduced speed of printing, particularly where electrostatic deflection is used. Printing speeds of a few KHz for alphanumerics are achievable with known drop-on-demand devices using deflection; however, reliable operation at speeds approaching 10 KHz and higher for alphanumerics has been difficult to achieve when electrostatic deflection is used. Where most continuous operation is feasible, higher speed reliability is rather good.
The previously mentioned Zoltan and Arndt patents discuss the desirability of providing a relatively high acoustic impedance looking from the ceramic transducer region of the ink jet into the supply conduit leading to the transducer region, so as to minimize the part of each transducer pulse inevitably used to drive ink from the transducer portion toward the reservoir. Analysis shows that perhaps 70% of the ink is ejected as a droplet, the balance being driven toward the reservoir. While operation without such a higher impedance is claimed by Zoltan, actual experience has shown that voltage pulses as high as 120 volts are required to produce droplets reliably and that frequency response is limited, particularly with electrostatic deflection, being reliable up to about 2.0 KHz. Even without deflection, reliable performance above 4 KHz has been hard to achieve. To improve performance, supply conduits have been carefully sized and their material chosen to provide appropriate inlet impedance. Operating voltages as low as 80 volts have been achieved by this means with good frequency response up to about 2 KHz, with deflection. However, the increased inlet impedance tends to slow down the refill of the jet following each pulse, so that operation at higher frequencies may result in depriming of the jet due to failure to refill quickly enough to be ready for the next pulse of the transducer. To offset partially these effects, the Zoltan type jets have been made with typical lengths of about one-half inch for the transducer and three-quarters inch for the jet liner. The requirements for particular inlet impedances and the overall jet size have created design complications for some applications, such as parallel and diverged arrays of jets.
Various forms of drop-on-demand ink jets have been disclosed heretofore, but glass lined capillary jets of the general type shown in FIG. 4 of the Zoltan patent have received considerable attention due to the corrosion resistance of the glass liner. Making such ink jets has presented a peculiar set of problems several of which are solved by the inventions disclosed in copending Ser. No. 886,882, filed Mar. 15, 1978 by the present applicant under the title Ink Jet Pen Assembly and Method. Another problem encountered in making glass lined jets concerns the capillary glass liners themselves, which must conform to rather rigid standards regarding geometry and material in order for the resultant jets to function properly. An acceptable material such as low lead KG12 glass made by Drummond Scientific Company of Broomall, Pennsylvania, has been known for some time; however, the control of jet geometry in the tiny liners has proven difficult, with the high reject rate and attendant high cost. A reliable method and means for making the glass liners has been lacking in the art.
Drop-on-demand ink jets and continuous streaming ink jets known in the prior art, produce very small ink droplets, typically in the range of 0.003 to 0.004 inch for drop on demand, and somewhat smaller for continuous streaming, based on applicant's experience. Such tiny drop sizes present problems when a continuous line is to be made by an ink jet imaging device; thus, considerable interest has been shown in reliably producing larger droplets.